Figure B: A LEO satellite turned off its downlink during this May 24 pass over Green Bank in preparation for the May 25 Phoenix Mars landing.

On March 13, 2008, a team from the NASA JPL Phoenix Mars Lander Project visited NRAO Green Bank to conduct a trial run in preparation for the May Entry, Descent, and Landing (EDL) phase of the Phoenix Mars Lander Mission. The Phoenix Lander has been designed to conduct studies of subsurface ice and soil composition near the Martian north pole.

The JPL team sent to Green Bank, headed by Peter Ilott, used the GBT to directly receive a UHF signal from the Phoenix Mars Lander on May 25. During the March 13 Operational Readiness Test (ORT-9), though, the GBT was looking for a signal at the same UHF frequency (401.56 MHz, topocentric) from the Spirit Mars Rover.

Just before Spirit crested the Martian horizon, very strong interference marched across the observational band, obliterating everything within about 30 kHz of its center frequency, which shifted about 20 KHz and then disappeared within about 11 minutes. About 30 minutes later, a second interferer with a similar signature was seen. Given the observed Doppler shift, these could only be Low Earth Orbit (LEO) satellites. A closer look at the data revealed a different modulation frequency for the two interferers, so it was clear that there were at least two orbiters that could potentially cause problems. How many more were there?

A comprehensive list of satellites by downlink frequency is not easy to come by (If you know of one, please send a link to interference@nrao.edu.) The GB RFI group  Carla Beaudet, Wesley Sizemore, Paulette Woody  consulted their spectrum management contacts at the NSF and at the quieter agencies that prefer not to be mentioned. Feiming Morgan, a NASA/JPL spectrum manager, also sought LEOs with downlinks between 401 and 402 MHz, finding one that was on a particularly critical interference path. Many 2-Line Orbital Element Sets later  2-Line Orbital Element Sets enable satellite orbit prediction  the GB RFI group was putting in odd hours monitoring satellite passes.

In the end, coordination with the Los Alamos National Laboratory and Sandia National Laboratories was critical to the GBT receiving the EDL signal from the Phoenix Mars Lander. Had this coordination not occurred, approximately 12 minutes of the critical 14-minute landing window would have been lost to interference. The Mars orbiters would still have received and relayed the Phoenix telemetry, but the Doppler information from which, e.g., Martian wind speeds can be deduced would have been lost, as would the GBT signal that was the first indication of a successful landing.